Methods for manufacturing a field-effect semiconductor device

ABSTRACT

A method of fabricating a field-effect transistor is disclosed. In one aspect, the method includes forming a channel layer comprising germanium over a substrate. The method additionally includes forming a gate structure on the channel layer, where the gate structure comprises a gate layer comprising silicon, and the gate layer has sidewalls above a surface of the channel layer. The method additionally includes forming sidewall spacers comprising silicon dioxide on the sidewalls by subjecting the gate structure to a solution adapted for forming a chemical silicon oxide on materials comprising silicon. The method further includes forming elevated source/drain structures on the channel layer adjacent to the gate structure by selectively epitaxially growing a source/drain material on the channel layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

Any and all priority claims identified in the Application Data Sheet, orany correction thereto, are hereby incorporated by reference under 37CFR 1.57. This application claims foreign priority to European patentapplication EP 12180025.4 filed on Aug. 10, 2012, the contents of whichis incorporated by reference herein in its entirety, and is herebyexpressly made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosed technology relates generally to methods of fabricatingfield-effect semiconductor devices, such as field-effect transistors(FETs), and more particularly to methods of fabricating field effectsemiconductor devices using a replacement gate process.

2. Description of the Related Technology

Scaling of semiconductor devices to integrate more devices per unit areacontinue to pose new challenges. In particular, for scaling offield-effect transistors (FET), as gate lengths continue to scale down,the structural design of offset spacers, sometimes referred to assidewall spacers, is becoming increasingly critical for transistorperformance. The desired dimensions of such dielectric offset spacersdesigned to meet certain performance targets are becoming increasinglysmaller and the corresponding process designs are becoming increasinglydifficult in order to achieve the desired critical dimensions.

Therefore, there is a need to address the increased sensitivity to gatespacer dimensioning using process techniques for gate sidewall spacerformation, particularly in manufacturing processes following a gate-lastapproach, sometimes referred to as a “replacement gate” process or a“damascene gate” process.

US patent application, for example, 2007/0287259 A1 discloses the use ofgate isolation spacers in a method of forming a semiconductor structureaccording to a replacement gate process.

Also, in US patent application 2006/0148182 A1, a self-alignedsource/drain quantum well transistor or high charge carrier mobilitytransistor is formed using a replacement metal gate process, in whichsidewall spacers temporarily bracket a dummy gate electrode.

Implant-free devices where source/drain (S/D) junctions are placed by anoffset spacer, and which comprise doped source/drain regions epitaxiallygrown with limited diffusion, e.g. extremely thin silicon on insulator(ET-SOI) devices and implant free quantum well devices (e.g. U.S. Pat.No. 7,915,608 B2), show a very strong sensitivity towards the offsetspacer critical dimension (CD). The offset spacer CD needs to beminimized to limit increase in external resistance and subsequentreduction in drive current, while it cannot be made too small to avoidhigh leakage between gate and drain. Careful process control istherefore desired. On 3D devices as for instance FINFET devices, this CDcontrol along the FIN sidewall is even more challenging.

A problem with the current techniques for fabricating FET devices isstill that they lack a precise control of the distance from thesource/drain extensions to the gate edge.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

It is an aim of the present disclosure to provide a method forfabricating a transistor device comprising a channel layer comprisinggermanium with improved offset spacer profile.

This aim is achieved according to the disclosure with a method showingthe technical characteristics of the first independent claim.

In one aspect, the method includes forming a channel layer comprisinggermanium over a substrate. The method additionally includes forming agate structure on the channel layer, where the gate structure comprisesa gate layer comprising silicon, and the gate layer has sidewalls abovea surface of the channel layer. The method additionally includes formingsidewall spacers comprising silicon dioxide on the sidewalls bysubjecting the gate structure to a solution adapted for forming achemical silicon oxide on materials comprising silicon. The methodfurther includes forming elevated source/drain structures on the channellayer adjacent to the gate structure by selectively epitaxially growinga source/drain material on the channel layer.

According to another aspect, the method further includes removinggermanium oxide formed on the channel layer.

According to another aspect, forming sidewall spacers and removinggermanium oxide are carried out simultaneously by subjecting the gatestructure and the channel layer to the solution.

It is an advantage that according to the above method, sidewall spacerscan be formed in the form of silicon oxide, at the height of the siliconor SiGe layer of a gate structure, while at the same time the channellayer comprising germanium is precleaned and prepared for epitaxialgrowth of elevated source/drain areas. An auto-alignment thus takesplace of the sidewall spacers. Moreover, the epitaxial growth of/for theelevated source/drain structures does not occur on the silicon oxidelayer. In typical prior art spacer technology, SiN is used for thesespacers, which can require the epitaxial growth to be selective withrespect to SiN, which is less evident. It will be appreciated by theskilled person that FET transistor devices comprising a channel layercomprising germanium are often provided in combination with a shallowtrench isolation (STI) structure which already comprises a silicon oxideand for which the epitaxial growth already has to be selective.

Advantageously, the method for fabricating a field-effect semiconductordevice according to embodiments allows better control and design of thedevice performance characteristics (e.g. resistance, capacitance andgate-drain leakage) by providing a mechanism to increase precisioncontrol for defining the source and drain region distance to the gateelectrode edges, e.g. for both overlap and underlap field-effectsemiconductor device design. In that sense, the method according to theinvention advantageously allows better repeatability of the field-effectsemiconductor device performance characteristics.

The method according to embodiments can be advantageously applied forfabricating both planar devices, such as, for example, implant-freequantum well (IFQW) FET devices or silicon on oxide (SOI) pFET devices,and non-planar FET devices such as, for example, FinFET devices.Advantageously, in case of non-planar devices, the S/D-gateoverlap/underlap distance along the FIN walls is more preciselycontrolled, and for example, a fixed external resistance (Rext) alongthe FIN walls is achieved.

The source/drain material can be undoped or in-situ doped. It cancomprise SiGe (e.g., for NMOS), Ge or GeSn (e.g., for pMOS) or any otherepitaxially grown layers known to be suitable to the skilled person.

In the following, when a reference is made to a silicon oxide layer, alayer is meant which comprises more than 90%, or more than 95%, or morethan 99%, or more than 99.9%, or more than 99.99% of silicon oxide.

The gate structure comprising silicon (e.g., silicon or SiGe layer),which can be a dummy gate structure, can be amorphous orpolycrystalline.

According to embodiments, providing the gate structure comprisesproviding a removable gate structure (RMG), comprising providing atleast one dummy oxide layer, providing a dummy silicon or silicongermanium layer, e.g. a polycrystalline silicon layer, and providing atleast one further dielectric layer.

According to embodiments, the method does not comprise providing alateral spacer structure for the gate structure by means of patterningand dry etching.

It is an advantage of embodiments of the present invention that no dryetching is required for spacer definition and that due to itsauto-aligning nature, alignment errors are non-existing.

According to embodiments, the method further comprises removing the atleast one dummy oxide layer, the dummy silicon or silicon germaniumlayer, and the at least one dielectric layer, and providing a high-kdielectric layer in a corresponding gate area.

It is an advantage that the distance between gate and source/drain canbe controlled perfectly. When applying a replacement gate process, thesilicon or silicon germanium layer and any dielectric layers of thedummy gate stack can be aligned with the thus formed silicon oxidespacers. Moreover, the use of a high-k dielectric can further provideappropriate characteristics to the FET transistor device.

According to embodiments, the solution is adapted for removing germaniumoxide, while not affecting silicon oxide.

According to embodiments, the solution comprises ozone and de-ionizedwater.

According to embodiments, the method further comprises subjecting thechannel layer comprising germanium and the gate structure to de-ionizedwater which is substantially ozone-free.

According to embodiments, the method comprises forming the silicon oxideon the sidewalls at a level of the silicon or silicon germanium layerwith a thickness larger than a single closed monolayer of silicon oxide.Preferably the thickness is smaller than about 3 nm, more preferablysmaller than about 2 nm, even more preferably smaller than about 1 nm.Larger thicknesses are not excluded.

According to embodiments, the selectively epitaxially growingsource/drain layer on the channel layer comprising germanium comprisesonly growing on the channel layer comprising germanium.

According to embodiments, the selectively epitaxially growingsource/drain layer on the channel layer comprising germanium does notcomprise growing on the gate structure nor on the oxide on the sidewallsof the gate structure.

According to embodiments, the method comprises providing further spacerstructures in between the elevated source/drain structures and the oxideon the sidewalls of the gate structure, after providing the elevatedsource/drain structures on the channel layer comprising germanium.

This provides the advantages of further tuning the properties of the FETtransistor device.

According to embodiments, the channel layer comprising germanium is afin structure of a fin-FET device or similar 3D transistor device.

It will be appreciated that the isotropic nature of the wet processingby means of the solution provides the advantage of generating a constantoffset between the doped source/drain areas and the channel all alongthe FIN sidewall.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be further elucidated by means of the followingdescription and the appended figures.

FIG. 1 illustrates a cross-sectional view of an intermediate structureof a field-effect transistor fabricated according to a first embodiment.

FIG. 2 illustrates a cross-sectional view of an intermediate structureof a field-effect transistor fabricated according to a secondembodiment.

FIG. 3 illustrates a cross-sectional view of an intermediate structureof a field-effect transistor fabricated according to a third embodiment.

FIG. 4 illustrates a cross-sectional view of an intermediate structureof a field-effect transistor fabricated according to a fourthembodiment.

FIG. 5 shows a top view SEM image of an intermediate structure of afield-effect transistor fabricated according to one embodiment, takenafter formation of source (S) and drain (D) on a silicon substratewafer.

FIG. 6 shows a cross-section TEM image of an intermediate structure of afield-effect transistor fabricated according to one embodiment, takenafter formation of source (S) and drain (D) on a silicon substratewafer.

DETAILED DESCRIPTION OF CERTAIN ILLUSTRATIVE EMBODIMENTS

The present disclosure will be described with respect to particularembodiments and with reference to certain drawings but the disclosure isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. The dimensions and the relative dimensions do notnecessarily correspond to actual reductions to practice of thedisclosure.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. The terms are interchangeable under appropriatecircumstances and the embodiments of the disclosure can operate in othersequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in thedescription and the claims are used for descriptive purposes and notnecessarily for describing relative positions. The terms so used areinterchangeable under appropriate circumstances and the embodiments ofthe disclosure described herein can operate in other orientations thandescribed or illustrated herein.

Furthermore, the various embodiments, although referred to as“preferred” are to be construed as exemplary manners in which thedisclosure may be implemented rather than as limiting the scope of thedisclosure.

The term “comprising”, used in the claims, should not be interpreted asbeing restricted to the elements or steps listed thereafter; it does notexclude other elements or steps. It needs to be interpreted asspecifying the presence of the stated features, integers, steps orcomponents as referred to, but does not preclude the presence oraddition of one or more other features, integers, steps or components,or groups thereof. Thus, the scope of the expression “a devicecomprising A and B” should not be limited to devices consisting only ofcomponents A and B, rather with respect to the present disclosure, theonly enumerated components of the device are A and B, and further theclaim should be interpreted as including equivalents of thosecomponents.

FIG. 1 is a cross-sectional view of a FET transistor device according toan embodiment of the disclosure at an early stage of manufacture,comprising a substrate layer SL with a channel layer or region CL, asource region S and a drain region D at opposite sides and adjacent tothe channel region, and a dummy gate DG, comprising a dummy dielectricDD. A silicon oxide layer OX covers the lateral sidewall SW1, SW2 of apolycrystalline silicon layer in the dummy gate structure and definesthe lateral sidewall SWO1, SWO2.

This device can be fabricated as follows, according to one embodiment:

A channel layer comprising germanium CL is deposited on a substratelayer SL, followed by a dummy dielectric deposition layer DD and thedeposition of the DG amorphous silicon or polycrystalline silicon layer(alternatively a SiGe layer can for instance be used). The DG layer issubsequently patterned using a dry etch process that stops on the DDlayer. The DD layer is subsequently removed selectively towards the DGand CL layer by either wet or dry processing. In case of a silicon oxidedielectric layer, DD can be removed by wet processing using hydrofluoricacid. At the point, the disclosure encompasses a cleaning step where theexposed silicon from DG is oxidized to a thin chemical SiO₂ layer OX,while the final GeO_(x) grown on the exposed germanium CL is minimized,such that the subsequent selective epitaxial growth of the source S anddrain D layer is only occurring on the exposed oxide-free Germanium CLlayer and is blocked on the oxidized DG top and sidewalls. Hereby it isassumed that the top of the dummy gate is provided with a protectingoxide layer at the same time that the silicon oxide is formed on thesidewalls of the dummy gate silicon or silicon germanium. Alternatively,a separate step of applying a silicon oxide hard mask can be applied. ontop of the silicon or silicon germanium of the dummy gate, as known inthe art.

In FIG. 2 depicts the same device, whereby spacers S11 and S12 areprovided on top of and/or adjacent or next to the oxidised sidewalls OXof the dummy gate structure, in order to further define the propertiesof the FET transistor device.

Spacers S11 and S12 are preferably made of a different material than DDand OX, so there is a sufficiently high selectivity of the wet removalof DD towards S11 and S12. These spacers S11 and S12 are deposited andanisotropically patterned after growth of the S and D layers. The mainpurpose of these spacers is to prevent a significant increase of thelateral size of the cavity left after the removal of the DG and DDlayers and sidewall silicon oxide OX (replacement gate flow).

FIG. 3 illustrates a similar embodiment, wherein rather than patterningspacers to control the lateral size of the DG cavity, a uniform layer L1can be deposited for the same purpose. This layer can for instancecomprise silicon nitride.

FIG. 4 illustrates another embodiment of the present invention after thereplacement gate processing. After removal of the DG, DD, and OX layersfrom embodiment disclosed with respect to FIG. 2, selectively to S11 andS12 (which can for instance be a silicon nitride), the cavity is filledup with the final gate stack. The final gate stack consists of adielectric layer DL, typically a high-k dielectric, and the finalconductive gate stack material GE. As such, a combination has been madewith a so-called removable gate, high-k last, flow. The dummy gate DG,including its gate dielectric DD and the oxide formed at the sidewallsof the silicon layer have been removed and are replaced by a high-kdielectric, defining sidewalls SWD1, SWD2. The final offset between theGE edge and the doped S and D layers is provided by the dielectricthickness D1. The well controlled deposition of the DL layer allows fora well-controlled underlap or overlap of the dopants under the gatestack GE, depending on the diffusion of the dopants during the rest ofthe processing.

FIGS. 5 and 6 show a top view SEM and cross-section TEM image aftergrowth of source (S) and drain (D) layers on a silicon substrate wafer.FIG. 6 corresponds to the embodiments described with respect to FIG. 1.In this experiment, layer SL is a thick relaxed SiGe layer, grownepitaxially on a silicon substrate. CL is a germanium layer depositedepitaxially on the SiGe SL layer. DD is a 5 nm thick silicon oxidelayer. The dummy gate layer DG is a 100 nm thick dry patterned amorphoussilicon layer. After wet removal of the DD layer, a ozone (O₃)/deionizedwater (DIW) cleaning step and a O-free DIW cleaning step were performed.This was followed by the epitaxial growth of a 10 nm thick S and Dgermanium layer. As described in the disclosure, the S and D layer isgrown only on the exposed CL layer, while no growth has occurred on theDG layer nor OX layer. The TEM image shows the presence of oxide at thesidewalls of the dummy gate structure.

In summary, due to the different material of the dummy polycrystallinesilicon (poly-Si) or SiGe gate and germanium channel, selectiveoxidation of the poly-Si or SiGe gate relative to the germanium activeregion can be applied before epitaxial source/drain deposition. Anexample is the use of an O₃/DIW rinse and preferably followed also by aO₃-free DIW wet rinse, which creates a chemical oxide on the silicon orSiGe gate but leaves the germanium surface germanium oxide free. In-situdoped epitaxially grown S/D can therefore selectively be grown towardsthe gate without the need of an off-set spacer encapsulation after thisO₃/DIW clean and optionally O-free DIW clean, as is typically employedin the prior art. This is especially attractive on 3D devices as forinstance FINFET devices, where the isotropic wet treatment allows for aconstant offset between the S/D epitaxial layers and dummy gate, allalong the FIN sidewall. In the subsequent RMG-high-k-last flow, thehigh-k deposition provides a well-controlled offset between S/D dopantsand channel.

A generic feature for embodiments of the present invention is the use ofa selective low temperature oxidation process prior to epitaxial growthon S/D regions. On the germanium channel device, an example of suchtreatment is a wet chemical clean in O₃/deionised water (DIW). Theselective oxidation of poly-Si relative to germanium results in nogrowth on the poly-Si gate and the oxide layer at its sidewall, while anin-situ doped epitaxial layer is grown on the germanium S/D regions. Assuch, an off-set spacer formation process is not needed to align the S/Ddoped epitaxial layer to the poly-Si gate.

In combination with a RMG-HKL flow, the minimal separation between theRMG metal gate material and S/D junctions is controlled by the depositedhigh-k thickness. The overlap between S/D junctions and gate can furtherbe enhanced by controlling the loss of the contact etch stop layer(CESL, L1) or 2^(nd) spacer material in combination with an optionaldiffusion of the S/D dopants. Preferably this loss due to non-perfectselectivity of the removal step does not remove more than 2 nm of thecontact etch stop layer (CESL, L1) or 2^(nd) spacer material in thelateral dimension or width direction of the gate structure.

What is claimed is:
 1. A method of fabricating a field-effect transistor(FET) device, comprising: forming a channel layer comprising germaniumover a substrate; forming a gate structure on the channel layer, thegate structure comprising a patterned gate layer comprising silicon, thegate layer having sidewalls above a surface of the channel layer;subjecting the gate structure and surfaces of the channel layer to asolution, wherein the subjecting selectively oxidizes the sidewalls ofthe gate layer to form sidewall spacers comprising a chemical siliconoxide thereon while preventing formation of germanium oxide on thesurfaces of the channel layer, thereby forming oxide-free surfaces ofthe channel layer laterally adjacent the gate layer; and formingelevated source/drain structures on the channel layer adjacent to thegate structure by selectively epitaxially growing a source/drainmaterial on the oxide-free surfaces of the channel layer.
 2. The methodof claim 1, wherein the subjecting further comprises removing germaniumoxide formed on the channel layer adjacent the gate layer.
 3. The methodof claim 2, wherein forming the sidewall spacers and removing germaniumoxide are carried out simultaneously by subjecting the gate structureand the channel layer to the solution.
 4. The method of claim 1, whereinforming the gate structure comprises forming a removable gate structure,comprising: providing at least one removable gate dielectric layer onthe channel layer; providing the gate layer that is a removable gatelayer on the removable gate dielectric layer; and providing at least onedielectric layer over the gate layer.
 5. The method of claim 4, whereinforming the sidewall spacers completes a spacer formation process suchthat the spacer formation process does not further include providing alateral spacer structure for the gate structure by dry etching.
 6. Themethod of claim 4, wherein forming the gate structure further comprisesremoving the removable gate structure, which includes removing the atleast one removable gate dielectric layer, the removable gate layer, andthe at least one dielectric layer, and wherein forming the gatestructure further comprises depositing a high-k dielectric layer over asurface formed by removing the removable gate structure.
 7. The methodof claim 2, wherein the solution is adapted for removing germaniumoxide, while substantially not removing silicon oxide.
 8. The method ofclaim 7, wherein the solution comprises ozone and de-ionized water. 9.The method of claim 8, further comprising further subjecting the channellayer and the gate structure to de-ionized water which is substantiallyozone-free.
 10. The method of claim 1, wherein forming the sidewallspacers comprises forming the chemical silicon oxide on the sidewallshaving a thickness larger than about a single monolayer of siliconoxide.
 11. The method of claim 1, wherein forming the sidewall spacerscomprises forming the chemical silicon oxide on the sidewalls having athickness less than about 3 nm.
 12. The method of claim 1, whereinselectively epitaxially growing the source/drain material comprises onlygrowing on the oxide-free surfaces of the channel layer.
 13. The methodof claim 1, wherein selectively epitaxially growing the source/drainmaterial does not include growing the source/drain material on the gatestructure nor on the sidewall spacers.
 14. The method of claim 6,further comprising forming additional spacer structures between theelevated source/drain structures and the sidewall spacers, after formingthe elevated source/drain structures and prior to removing the removablegate structure.
 15. The method of claim 1, wherein the FET is a fin-FETtransistor device comprising a fin-shaped channel structure, wherein thechannel structure comprises the channel layer.
 16. The method of claim14, further comprising removing the sidewall spacers after forming theadditional spacer structures and prior to removing the sacrificial gatestructure.
 17. A method of fabricating a field-effect transistor,comprising: forming on a substrate a channel layer comprising germanium;patterning a sacrificial gate layer comprising silicon on the channellayer; exposing sidewalls of the patterned sacrificial gate layer to asolution to form thereon sacrificial sidewall spacers comprisingchemical silicon oxide; forming an elevated source and an elevated drainon exposed surfaces of the channel layer laterally adjacent thepatterned sacrificial gate layer; after forming the elevated source anddrain, forming permanent sidewall spacers on the sacrificial sidewallspacers; removing the sacrificial gate layer and the sacrificialsidewall spacers, thereby forming a cavity comprising an exposed channelsurface and exposed inner surfaces of the permanent sidewall spacers;and depositing a permanent gate dielectric layer on the exposed channelsurface of the cavity and depositing a permanent gate metal on thepermanent gate dielectric layer.
 18. The method of claim 17, whereinexposing sidewalls of the patterned sacrificial gate layer to thesolution also exposes surfaces of the channel layer adjacent thepatterned sacrificial gate layer to the solution, wherein exposingselectively oxidizes the sidewalls to form the chemical silicon oxidewhile preventing formation of germanium oxide on the channel layer,thereby forming oxide-free surfaces of the channel layer laterallyadjacent the patterned sacrificial gate layer.
 19. The method of claim18, wherein forming the elevated source and the elevated drain comprisesgrowing by selective epitaxy on the oxide-free surfaces of the channellayer laterally adjacent the patterned sacrificial gate layer.
 20. Themethod of claim 18, wherein the solution comprises O₃ and deionizedwater.